1. Field of the Invention
The present invention relates to objective lens for optical pick-up devices, optical pick-up devices, and optical information recording and/or reproducing devices.
2. Description of the Related Art
In the recent years, due to high densities of optical disks (optical information recording media), the objectives lenses with high numerical aperture (NA) numbers are being used in order to meet the demand for making the light focus spot smaller in the optical pick-up devices used for recording and/or reproducing information in optical disks.
For example, in an optical pick-up device for high-density optical disks using a blue-violet semiconductor laser at a wavelength of 405 nm as the light source, it has been proposed to use a numerical aperture number NA of 0.85 in order to achieve high density.
As an objective lens with an NA of 0.85, a 2-component plastic lens has been proposed in Japanese Unexamined Patent Application Laid Open No. 2003-248956 in which the refractive force on the incident light beam is divided between two lenses and the NA of 0.85 is achieved by relaxing the manufacturing tolerances of the individual lenses, and in which the change in the spherical aberration due to heat radiated from the actuator or due to changes in the external temperature of the optical pick-up device has been suppressed to a small value.
In an optical pick-up device, generally, the focusing and tracking are carried out by moving the objective lens in the direction of the optical axis and in the radial direction of the disk by actuators. At that time, the objective lens is driven by passing a current through the focusing coil or through the tracking coil and by the resultant magnetic force between the magnetic field generated by the coil and a magnet.
However, when using a plastic for high NA number lens and driving the lens in the above manner, a problem occurs that has not been present in the previous structures. Since it is difficult in designing the actuators to provide or place the focusing coil or the tracking coil in a circularly symmetrical manner with respect to the optical axis of the objective lens, during the operations of the actuators, a temperature distribution that is not circularly symmetrical with respect to the optical axis inside the objective lens, and as a consequence of that, the astigmatic aberration of the objective lens changes. In particular, when the objective lens has a structure with multiple components, since the refractive surface increases for an optical beam of NA 0.85, there is very small change in the heights at which the optical beam passes through within each of the respective objective lenses, and hence it is easily affected by the changes in the refractive index associated with the temperature distribution. In addition, if the speed of recording and/or reproducing information is made high, it will be necessary to pass large currents through the focusing coil and the tracking coil, which increases the heat generated by the coils. As a consequence, the changes in the astigmatic aberration mentioned above become large, which has bad effects on the optical disk recording and/or reproducing characteristics. Further, normally, even when the external diameter of the objective lens is large and its mass is also large, it is necessary to pass large currents through the focusing coil and the tracking coil, and hence the changes in the astigmatic aberration become large.
However, the technology disclosed in Japanese Unexamined Patent Application Laid Open No. 2003-248956 is a technology that compensates for the changes in the spherical aberration that occurs in an objective lens when the temperature distribution is uniform, and there is no mention there about the problem of deterioration in the astigmatic aberration caused by non-uniform temperature distribution within the objective lens, such as, for example, due to the heat generated by the current passed through the focusing coil or through the tracking coil as described above, and, of course, also no countermeasures have been taken for such problems in the technology disclosed in this document.